Hyperglycemia worsens Insulin resistance in skeletal muscle in both Type 1 and Type 2 diabetic patients, and high glucose induces defects in insulin-stimulated glucose transport in target cells. Glucose-induced insulin resistance (`glucose toxicity') requires glucose metabolism via the hexosamine biosynthetic pathway; however, despite 2 decades of study, the molecular link between hexosamine pathway metabolism and defects in insulin action remain unknown. We have reported that muscle TRIB3 is inversely correlated with insulin sensitivity and positively correlated with fasting glucose in human patients is induced by glucose in a manner that is dependent upon the hexosamine pathway, and mediates defects in insulin-stimulated glucose transport in muscle. In addition to its pathological role in chronic hyperglycemia, we have also defined a novel regulatory role for TRIB3 in the acute regulation of nutrient metabolism under conditions of nutrient excess and fasting. We will now identify mechanism by which glucose induces TRIB3 expression and determine whether TRIB3 mediates glucose induced insulin resistance and regulates fuel metabolism in humans and genetically manipulated mice. To test these hypotheses, our specific aims will be: Aim 1: Identify the mechanisms (cis elements and trans factors) by which glucose induces TRIB3 gene expression. Aim 2: In muscle-specific transgenic and knockout mice, examine whether (i) glucose-induced insulin resistance in diabetes and (ii) the metabolic effects of nutrient deprivation and excess, are mediated in a TRIB3 dependent manner. Aim 3: Assess contribution of muscle TRIB3 in regulating insulin sensitivity and metabolism, both systemically and at the level of skeletal muscle, in: (i) T2DM patients before and after euglycemic therapy to examine glucose-induced insulin resistance, and (ii) in subjects during hypocaloric feeding and stable weight loss. Thus, we will elucidate fo the first time a pathophysiological role for TRIB3 as a mediator of glucose- induced insulin resistance in diabetes, and the ability of TRIB3 to act as an acute physiological regulator of glucose transport and mitochondrial oxidation in the context of available intracellular fuel stores during periods of fasting and nutrient excess. These data in cultured cells, genetically-manipulated mice, and human subjects will elucidate a novel pathophysiological role for TRIB3 in glucose-induced insulin resistance, and a novel physiological role in the regulation of fuel metabolism. These studies will develop TRIB3 as a new target for therapy in diabetes.